1. Field of the Invention
The present invention relates to an image forming apparatus provided with at least two rollers arranged between a transfer unit for transferring a toner image onto a recording sheet, and a discharge port adapted for discharging the recording sheet transported from the transfer unit outside of the image forming apparatus.
2. Description of the Related Art
Generally, in image forming apparatuses such as a printer and a copier, an imaging unit with a photosensitive drum is widely used. In such an image forming apparatus, an electrostatic area is formed on the surface of the photosensitive drum by a charger, an electrostatic latent image is formed on the electrostatic area based on image data generated by laser light or the like irradiated from an exposure unit, and the electrostatic latent image is developed into a toner image by a developing unit. Then, the toner image is transferred onto a recording sheet by a transfer unit, followed by removal of toner residues from the surface of the photosensitive drum by a cleaning unit. After the transferred toner image is fixed onto the recording sheet by application of heat by a fixing roller provided in a fixing unit, the recording sheet carrying the fixed toner image is discharged outside of the image forming apparatus by a discharge roller.
In thermal fixation of the toner image, it is highly likely that moisture contained in the recording sheet may evaporate, thereby causing undulation of the recording sheet. In order to solve such a drawback, for instance, Japanese Unexamined Patent Publication No. 2004-35177 discloses a technique of flattening a recording sheet after an image fixation by setting the transport speed of the discharge roller higher than the transport speed of the fixing roller and applying a tension to the recording sheet.
Further, in recent years, there is known an image forming apparatus equipped with a so-called “short path mechanism”, wherein the length of a transport path for transporting a recording sheet is shortened. In the image forming apparatus equipped with the short path mechanism, there is a case that merely a discharge roller is provided on the downstream side of the transport path relative to the fixing roller, and the transport path between the fixing roller and the discharge roller is curved.
If the arrangement disclosed in the publication is applied to the image forming apparatus equipped with the short path mechanism, it is highly likely that the recording surface of the recording sheet for image formation may be contacted with the inner wall of the curved transport path between the fixing roller and the discharge roller because the transport speed of the discharge roller is set higher than that of the fixing roller for the purpose of applying a tension to the recording sheet. The contact of the recording surface of the recording sheet with the inner wall of the curved transport path may likely cause adhesion of the fused toner that has just been fixed to the recording sheet to the inner wall of the transport path. As the image formation is repetitively carried out, the adhered toner may be accumulatively deposited, with the result that the deposited toner may clog the transport path, and resultantly cause a sheet jam.
There is an idea of relaxing a recording sheet while the recording sheet is being transported along the transport path between the fixing roller and the discharge roller by setting the transport speed of the discharge roller lower than the transport speed of the fixing roller. However, in the conventional art, the fixing roller and the discharge roller are constantly driven at the same speeds once the respective transport speeds thereof are determined. Therefore, in case of transporting a recording sheet of A3 size at the transport speed of transporting a recording sheet of A4 size, for instance, there is likelihood that the recording sheet of A3 size may be relaxed too much owing to a longer length of the A3 size recording sheet than the A4 size recording sheet, with the result that the recording sheet of A3 size may be undulated while being transported between the fixing roller and the discharge roller, and discharged in an undulated state.